1. Field of the Invention
The invention relates to an optical coupling circuit element and more particularly to an optical circuit coupling element using a micro Fresnel lens for collimating light emitted from a coherent source such as a semiconductor laser to an optical communication means such as an optical fiber for condensation.
2. Description of the Prior Art
Optical fiber communication for a long distance and high capacity uses semiconductor lasers having higher coherency as a source. This is done since light emitted from the semiconductor laser is required to be effectively directed to optical communication means such as an optical fiber.
A far field pattern of light emitted from semiconductor laser is usually longer lengthwise and elliptic. Such light when collimated, by use of a customary axially symmetric optical lens, will be gathered elliptically, so that when the light is input to a core of an optical fiber whose aperture angle is in right circular conical shape, a poorer in coupling efficiency will occur in comparison with a circular collimation. To solve the problem, various constructions have been proposed previously.
The use of a micro Fresnel lens with a hologram on a transparent substrate for an optical circuit coupling element has been hitherto proposed (Japanese Patent Publication No. SHO 47-43670). The micro Fresnel lens realized reduction of spherical aberration that is difficult for the conventional lens to achieve, and lowering of ellipticity of elliptically emitted light from semiconductor laser used as a source. In recent years, a computer generated hologram has been used for the fabrication of micro Fresnel lens, wherein in a similar manner for production of IC, a hologram pattern is formed by exposure on a transparent substrate which is then diced similarly to an IC chip. The computer designed hologram can be formed by writing on an EB resist with an electron beam. By changing the dosage of the electron beam, a saw-tooth shaped diffraction grating can be achieved and a micro Fresnel lens having diffraction efficiency of almost 100% can be fabricated.
FIG. 10 shows a conventional example using a micro Fresnel lens to direct the light emitted from semiconductor laser to an optical fiber. A micro Fresnel lens 13 formed on a transparent substrate 7 can result in an outgoing of elliptic light (spherical wave) 5a from a semiconductor 5 in a manner of having a round shape on the section of an optical fiber 6. In detail, the light passing the micro Fresnel lens can be converted into a spherical wave 5b collimating circularly to a core 6a at the optical fiber 6. In other words, the spherical wave converges into a shape of cone S in which a right circular cone having as its top the center A of the core 6a of the optical fiber 6 is cut with the micro Fresnel lens [see FIG. 10(b)]. Thus, the micro Fresnel lens 13 is arranged in such a manner that the mother line m of the core 6a of optical fiber 6 is placed on an extension of a line 1 connecting a center of circle defined by the bottom of that right circular cone with a top thereof (the line 1 will be called hereunder "axis of right circular cone").
When using the micro Fresnel lens 13 shown in FIG. 10, the optical axis at the outgoing side, i.e., the axis of right circular cone does not correspond to the optical axis at the incidence, i.e., the axis of elliptic incident ray. When such an off-axial type construction is used, the following problems will occur. First, since the arrangement is poor in symmetry, the optical axis is likely to be deflected due to thermal expansion of mount material, etc. Also, locations or angles in setting modules are hard to be decided. Furthermore, the optical system cannot be made compact as a whole.
To solve the abovesaid problems, two micro Fresnel lens may be used to enable an arrangement having a favorable symmetry, and such a coaxial type construction that the optical axis at the incidence side are coaxially disposed with the optical axis at the outgoing side, i.e., the axis of the right circular cone.
However, the coaxial construction with two micro Fresnel lens causes an additional problem. Generally, as an optical system increases the number of constituent optical elements, accuracy of the entire system will lower due to cumulation of specific positional deflections of the optical elements. That is, in the case of using two micro Fresnel lens, there is the problem of positional deflection between the two lens.
The present invention has been designed to overcome the abovesaid problems.